This is a study to compare dual and single protease inhibitor salvage antiretroviral therapies for HIV infected subjects who have virologic evidence of nelfinavir treatment failure and who are niave to non-nucleoside reverse transcriptase inhibitors and to both components of at lease one of the following nucleoside reverse transcriptase inhibitor combinations: ZDV and 3TC; d4T and 3TC; d4T and ddI or ZDV and ddI. Eligible subjects will be randomized in an open-label fashion to the 4 treatment arms. Plasma HIV RNA will be quantified in real time and the study monitored closely for early virologic endpoints. An interim analysis will be performed after 40 study endpoints have occurred to identify underperforming or poorly tolerated treatment arms. Interim analysis will be repeated after each additional 40 endpoints. For both studies, the GCRC will be used for drug dosing and blood collection.